Hi No1egg.
children have new behaviors all the time, block the side vision with an open hand by the side of his face to redirect him. Have something he likes to look at like a toy etc. Show him and say see look at the car!
I know more people will reply to your post with some good suggestions. what works for one may not work for another.
but try my suggestion it works well :)

Well, the good old side stare. I really am not so sure what to tell you. I just try and get my guy to engage with me. I give the old line hey look at me. Denise

Check out the discussion on periferial vision a few days back, Kris shared her insights on how it can cause both benefit and overstim as well. I remember being fastinated by overhead things when I was little. For my kid the issues are about the obsessive need to do it, rather than the balancing or stim relationship to the activity. Look at all the info and give the observation to your OT for feedback if you are fortunate to have a good resource. Try out behavior modifications but I think too heavy is not good , maybe if a big problem just avoid where the line runs and see it he picks on another object.